In the past, when we've dealt with fleas (we had dogs that I groomed for other people bring them in far too often, argh!) we got containers of liquid...and dropped them in it while flea combing. You can use water, or soapy water, but sometimes they can spring off of it. In that case, alcohol works better.

Or, if you had an old Ziploc baggie, you can put all of the fur you comb out of the dog in that, as you go, fleas and all, and then zip it up before throwing away. They can't get out. When there are fleas, I take no prisoners, all's fair in flea wars!

The D sounds like a good plan.

Vitamin E works really well, too. You can mix it with the coconut oil, or put it on first, then put on the coconut oil.